The invention relates to a filter comprising a shell construction and at least one resonator in the shell construction, the resonator having a turning point where the coaxial resonator turns backwards and in which filter the coaxial resonator is attached to a connection surface included in the shell construction.
Radio-frequency filters, such as resonator filters are used for implementing high frequency circuits in base stations of mobile telephone networks, for example. Filter constructions can be used, for example, as interface and filtering circuits in the amplifiers of transmitter and receiver units in base stations.
There are several different types of resonator filters comprising a shell construction, or a body: e.g. a coaxial resonator filter and an L-C filter. The present solution relates to coaxial resonators. In addition, for example, a helix resonator and a cavity resonator construction are known. All these resonator types comprise a metallic shell construction. In coaxial resonator constructions, for example, the shell envelops a conductor which is positioned in the middle of the shell and which is called a resonator or a resonator pin. In helix resonators the wire of the resonator is wound into a spiral coil. A cavity resonator only comprises a cavity.
As the size of the equipments requiring filters has become smaller, it has become necessary to make the resonator small-sized. To reduce the space required by the resonator, a helix coil is used where the same operational length will be in a shorter space because the resonator in the helix resonator has been formed as a coil. A helix coil is, however, difficult to manufacture, and a further disadvantage is that it very difficult to attach to the helix coil a wiring connection or other such projection which is needed when the switching between two resonance circuits is to be adjusted. A further problem with helix resonators is that it is difficult to support them and carry out temperature compensation. References FI-80163, FI-80811 and FI-90157 disclose supports of helix resonators where the annular lower edge of the helix resonator coil rests on the surface to which the helix is attached. But as mentioned, it is difficult to support a helix resonator and the manufacturing of the actual helix is difficult in comparison to a bar-like coaxial resonator.
In coaxial resonators, a resonator is normally a straight pin which is connected only to the bottom of the resonator. This type of resonator is long and thus takes up a lot of space.
A coaxial resonator type, which is U-shaped, that is, comprises a turning point, is also known. Such a construction allows a smaller size but its manufacturing is problematic because the connection of the initial section and the support of the end section of the resonator will be on different surfaces wherefore the manufacture and installation of the filter will become considerably more difficult.